Amyotrophic lateral sclerosis (ALS) is a degenerative motor neuron disease that occurs in sporadic and familial forms (FALS). the most common inherited form, accounting for about 10% of all ALS cases, is autosomal dominant (FALS-AD). There is also an autosomal recessive form (FALS-AR). We have identified 162 families with FALS-AD and 3 with FALS- AR. We have generated lymphoblast lines from over 900 members of these pedigrees. With Dr. T. Siddique (Chicago), Dr. G. Rouleau (Montreal) and Drs. R. Horvitz (M.I.T.) and J. Gusella (M.G.H.) in Boston, we have found that a subset of the FALS-AD families is linked to the mid-region of the long arm of chromosome 21. We have also recently obtained a lod score of 6.2 for a locus on chromosome 2 in a single FALS-AR family. The purpose of this project is to refine the genetic mapping of the loci for these diseases, define a physical map of the linked regions, and identify the mutant genes and aberrant gene product(s) for these disorders. Genetic mapping will continue using conventional linkage analysis with PCR amplification of di- and trinucleotide repeats as well as candidate genes and anonymous DNA markers. Physical mapping will employ several strategies using (1) contiguously mapped cosmids and YACs cloned using chromosome 2 and 21-linked markers and (2) long range mapping with pulse field gel electrophoresis. Through the laboratory of J. Gusella, we currently have access to approximately 102 YACs from chromosome 21 (of which 52 are associated with the linked region). These investigations should prove significant in at least three respects. They will (1) provide insight into the molecular basis of familial and sporadic ALS and potentially lead to rational therapies; (2) contribute to the development of physical maps of chromosomes 21q and distal chromosome 2q; and (3) provide an unusual opportunity to employ homozygosity mapping as a novel strategy for linkage analysis in the FALS-AR families.